1. Field of the Invention
The present invention relates to protective jacket material compositions with improved physical and electrical performance characteristics.
2. Description of Related Art
Electro-magnetic energy conductors, whether transmitting electrical current or light signals, are conventionally coated with a protective jacket. Such jackets may serve one or more of a host of functions, including providing electrical and/or thermal insulation, serving as a sheath to assist in containing multiple conductors, and providing physical protection for the conductor from attack or damage from environmental conditions or stresses applied to the conductor during use.
Among the successful jacket materials generally used today are polyvinyl chloride (PVC), polyurethane, polyimide, polytetrafluoroethylene (PTFE), expanded PTFE, fluorinated ethylene propylene (FEP), perfluoroalkoxy polymer (PFA), polyesters, silicone rubber, and nylon. These materials may be applied over the conductors in a variety of ways, including by extrusion, tape wrap, insertion within pre-formed tubes, shrink wrap. etc. The choice of material or materials and the manner or manners of application over a conductor are all design choices heavily dependent upon the required properties sought and the conditions anticipated for the conductor while in use.
In many cases, an additional physical structure may be incorporated into the jacket material or applied over it to impart improved physical characteristics. For instance, wire, glass fibers, polymeric fibers, and the like may be applied over a jacketed conductor to provide greater longitudinal strength or other physical characteristics not provided by a jacket alone. At other times, material may be combined to form a composite structure to provide such properties (e.g., silicone coated glass fiber braid, foil laminated polyesters, etc.).
Unfortunately, producing a correct balance between different properties may be extremely difficult for many applications. For example, it is often necessary to have a cable with a great deal of flexibility while being sufficiently strong to resist elongation or breakage if longitudinal strain is applied to the conductor. The use of a longitudinally strong jacket to relieve such stresses on a conductor is commonly referred to as "load sharing." Although it is a relatively simply matter to reinforce a conductor to produce good stretch resistance, typically such reinforcement significantly reduces other properties such as a wire's flexibility. With the use of very fine conductors (e.g., on the order of less than 0.25 mm), load sharing may be very important but must be balanced against loss of the flexibility of such wires.
Achieving an acceptable balance between these properties is often not possible. While a jacket of silicone has a high degree of flexibility, it provides very poor resistance to elongation, thus contributing virtually no load sharing to longitudinal forces. The reinforcement of such materials with wires or fibers imparts the needed longitudinal strength, but tends to simultaneously make the wire far less flexible.
Another concern often encountered is that many conductors are subjected to extremely harsh environmental conditions that can weaken or destroy conventional jacket materials. For instance, in medical applications it is commonly necessary to sterilize cabling with steam at high temperatures and pressures (e.g., in an autoclave) and/or with harsh chemicals. Many otherwise suitable materials are incapable of withstanding such treatments. Silicone and polyurethane are notoriously incapable of withstanding high temperature treatments and may be subject to degradation by certain chemicals. For example, silicone jacketed conductors tend to expand significantly during steam sterilization, requiring the use of a reinforcement material to avoid over-expansion and damage to the cable.
Porous, expanded PTFE, such as that made in accordance with U.S. Pat. No. 3,953,566 to Gore, has excellent dielectric properties and functions extremely well as a cable jacket for most applications. Among its desirable attributes are excellent strength and flexibility, high temperature resistance, and chemical resistance. Unfortunately, the porous nature of expanded PTFE may allow certain harsh chemicals (e.g., gluteraldehyde) to penetrate its interstices. At a minimum, this may result in undesirable wetting of the expanded PTFE jacket material. At worse, such chemicals can alter the properties of the jacket material (e.g., making the jacket less flexible) or even diminish performance of the cable by causing de-lamination or by attacking or interfering with the conductor itself. This risk of conductor damage may be of particular concern where exposure to such chemicals is combined with repeated high temperature and pressure stream sterilization treatments in autoclave cycling.
Another problem with a number of existing jacket materials is that they are too often limited in their handling requirements. Even extremely effective jacket material like expanded PTFE would be significantly improved if it could be produced with certain improved elastic properties. An expanded PTFE with improved axial elasticity may impart better abrasion resistance, improved cut-through resistance, and more forgiving handling characteristics.
It is accordingly a primary purpose of the present invention to provide a jacket material for conductors that provides good load sharing along the longitudinal length of a wire while contributing minimal resistance to flex.
It is a further purpose of the present invention to provide a jacket material that can withstand extremely rigorous use conditions, such as sterilization procedures, without compromise of the jacket or the conductor.
It is still another purpose of the present invention to provide a jacket material with a variety of improved handling properties, increasing both the range of possible uses for the jacket material and the ease of applying and using such material.
These and other purposes of the present invention will become evident from review of the following specification.